1. Field of the Invention
The present invention relates generally to aircraft flight path design, and more particularly to missed approach procedure design.
2. Description of the Related Art
There are two basic sets of rules for flight operations, Visual Flight Rules (VFR) and Instrument Flight Rules (IFR). Visual Meteorological Conditions (VMC) are those weather conditions in which pilots have sufficient visibility to maintain visual separation from terrain, obstacles, and other aircraft. Instrument Meteorological Conditions (IMC) are those weather conditions in which pilots cannot maintain visual separation from terrain, obstacles, and other aircraft.
Under Visual Flight Rules (VFR), the pilot maintains separation from terrain, obstacles, and other aircraft by visual reference to the environment surrounding the aircraft. The guiding principle for VFR is “See and Avoid”. Under Instrument Flight Rules (IFR), the pilot maintains separation from terrain and obstacles by reference to aircraft instruments only. The guiding principle for IFR is “Positive Course Guidance” to track a “hazard-free path” which provides separation from terrain and obstacles. Separation from other aircraft is provided by Air Traffic Control. VFR principles may only be used under VMC; however, IFR principles are used under both VMC and IMC.
The simplest form of IFR operations is dead-reckoning where the pilot navigates using only magnetic heading, airspeed, and time. This allows the pilot to estimate his/her location by using a map to identify a starting point then using heading, speed, and time to determine distance and direction traveled from the starting point. Dead-reckoning is highly inaccurate in windy conditions because the pilot cannot accurately determine the actual ground speed or aircraft track (which differ from airspeed and heading due to the velocity of the wind). Modern inertial navigation systems automate the dead-reckoning process and provide much higher accuracies than the pilot can achieve without assistance. However, even the best, and most expensive, inertial navigation systems suffer from position errors that increase over time (typically with a drift rate of 2 nautical miles or more per hour).
Various navigational aides (NAVAIDS) have evolved over time to improve the accuracy of navigation in IMC. The first generation of NAVAIDS includes ground-based navigation radio systems such as VHF Omnidirectional Range (VOR), Distance Measuring Equipment (DME), and Instrument Landing System (ILS). These solutions allow an airborne radio receiver to determine either bearing to a ground-based transmitter (e.g. VOR) or distance to the transmitter (e.g. DME). The ILS is a specialized system that allows the airborne radio receiver to determine angular deviation from a specific bearing from the transmitter (Localizer) and specific descent path (Glide Slope). While these systems provide significant improvement in accuracy over inertial navigation systems, they require very expensive ground infrastructures which limit the number of locations where they may be installed.
Another disadvantage of ground-based radio positioning systems is that such systems provide less certainty of an aircraft's position the farther the aircraft is from the transmitter. Recognizing this limitation, regulators have established a set of criteria for building instrument-based navigational procedures called TERPS (Terminal Instrument Procedures) for designing approaches that recognize the limitations of the technology. TERPS employs trapezoidal obstacle identification surfaces that take into account inaccuracies in the aircraft's positional certainty. TERPS is formally defined in US FAA Order 8260.3B, along with associated documents in the 8260 series. The international equivalent of TERPS is called PANS-OPS, promulgated by the International Civil Aviation Organization (“ICAO”) (document 8168); the two combined represent virtually 100% of conventional approaches in place today. Procedures developed in accordance with the TERPS or PANS-OPS have serious limitations in that they are written using “lowest common denominator” aircraft performance expectations. The smallest general aviation aircraft and the largest transport jets all use the same procedures to depart and arrive at terrain-challenged airport in IMC regardless of the capabilities of the aircraft or aircrew.
The next generation of NAVAIDS exploits the Global Positioning System (GPS) infrastructure which was deployed by the Department of Defense. Airborne Satellite Navigation (SATNAV) receivers can calculate the current position of the aircraft to far greater accuracy than can be achieved with VOR and DME and can provide similar performance to ILS near the runway threshold.
An emerging model for IFR operations defines operating procedures based upon the concept of Required Navigation Performance (RNP). Instead of defining approach and departure paths based upon the lowest accuracy of the available NAVAIDS, RNP defines the minimum performance requirements that an airborne system must achieve to use a published RNP procedure. In addition, a new paradigm is emerging that allows RNP procedures to be developed and published that assume Special Aircraft and Aircrew Requirements (SAAAR). Even though RNP-SAAAR procedures are published (and therefore public), they may only be used by aircraft operators that have been authorized in advance by the regulatory authorities. These RNP-SAAAR procedures will allow complex approach and missed approach procedures at terrain-challenged airport in IMC; however, there are hundreds of terrain-challenged airports around the world, and it will be a long time before procedures are developed and published for all the airports. In fact, it may be too expensive to develop RNP-SAAAR procedures for small airports that have very low utilization.
Thus, as discussed above, the TERPS defines the criteria for the creation of arrival procedure from top of descent through a successful landing or a missed approach.
The missed-approach point is the location along the approach path that the pilot must decide to continue the landing or to go around. Precision approaches have a Decision Height (DH) where the pilot must decide to land or go around. Non-precision approaches have a Minimum Descent Altitude (MDA) (i.e. lowest published descent altitude), where the pilot must have visual reference to the airport to proceed. Decision heights range from 0 feet above the runway (Cat IIIc) to 200 feet (Cat I) while minimum descent altitude range from hundreds of feet to thousands of feet above the runway.
FIG. 1 (Prior Art) shows the situation where terrain along the non-precision approach path requires the MDA to be substantially higher than the typical Decision Height on a precision approach. Since the airborne navigation systems may not have enough certainty in the determination of aircraft position to ensure clearance of the terrain, the pilot must be able to see any terrain that protrudes near to the desired descent path to the runway.
In this case, if a means can be provided to allow the pilot to have an alternative means to achieve situation awareness of the terrain along the approach path, it may be possible for the pilot to descend below the MDA to an altitude more typical of a Cat I DH.
FIG. 2 (Prior Art) shows the situation where terrain along the missed approach path requires the MDA on the non-precision approach path to be substantially higher than the typical Decision Height on a precision approach. In this case, the pilot must be able to safely perform a go-around if there is not sufficient visibility to see the runway environment. As will be disclosed below, this invention is primarily intended to provide a safe extraction path for situations where the MDA is determined by terrain and obstacles along the missed approach path. However, the inventive concepts herein may be paired with other solutions that address MDA determined by terrain and obstacles along the non-precision approach path.
The TERPS use a one-size-fits-all approach to defining MDAs and approach paths. The TERPS uses a modest climb requirement of 200 feet per nautical mile to ensure that the aircraft can climb above terrain along the missed approach path. Many modern small jets can easily outperform this climb requirement even with an engine out condition. However, published approach/miss-approach procedures are limited to the lowest performing aircraft type.
In this case, if a means can be provided to allow the pilot to verify the climb performance of the aircraft on a given day under given atmospheric conditions, then it may be possible to descend below the MDA and successfully complete a missed approach.
FIG. 3 (Prior Art) shows a worst case situation of a one-way airport where arrivals and departures may only use a single course (heading) into or out of a terrain challenged airport. This means that missed approach procedures are limited to paths that allow the aircraft to safely reverse course and leave in the opposite direction to the approach.
In this case, the aircraft cannot descend below MDA unless some means is provided to ensure that the pilot has situation awareness of all terrain and obstacles in the terminal area as well as a means to verify that the aircraft has sufficient climb performance to climb to a safe operating altitude.
Constraints:
It is highly desirable to find a means to allow an aircraft to descend below published MDAs to increase the probability that the flight can proceed to a successful landing instead of the flight diverting to an alternative airport.
It is also desirable to avoid multiple solutions to handle the various situations (listed above) that depend upon the location of hazards (terrain and human-made obstacle) in the terminal area. One common solution for all terrain-challenged airports minimizes implementation and certification costs as well as training costs once the solution is fielded.
As will be disclosed in detail below, the present patent application covers one such solution for generating a hazard-free missed approach path.
U.S. Pat. No. 7,302,318, entitled “Method for Implementing Required Navigational Performance Procedures” issued to D. J. Gerrity et al, discloses a method for designing an approach for a selected runway. The method includes gathering data regarding the height and location of all obstacles, natural and man-made, within an obstacle evaluation area. A preliminary approach path is laid out for the runway, including a missed approach segment, and a corresponding obstacle clearance surface is calculated. In the preferred method the obstacle clearance surface includes a portion underlying the desired fixed approach segment, and may be calculated using a vertical error budget approach. The obstacle clearance surface includes a missed approach segment that the aircraft will follow in the event the runway is not visually acquired by the time the aircraft reaches a decision altitude. A momentary descent segment extends between the first segment and the missed approach, and is calculated on physical principles to approximate the projected path of the aircraft during the transition from its location at the decision altitude to the missed approach segment. The preliminary path is then tested to insure that no obstacles penetrate the missed approach surface, and may be improved, e.g. lowering the decision altitude, by adjusting the obstacle clearance surface until it just touches an obstacle.